drm/amdgpu: derive GTT display support from DM

Rather than duplicating the logic in two places,
consolidate the logic in the display manager.

Acked-by: Yifan Zhang <yifan1.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_mode.h

@@ -341,6 +341,7 @@ struct amdgpu_mode_info {
 	int			num_crtc; /* number of crtcs */
 	int			num_hpd; /* number of hpd pins */
 	int			num_dig; /* number of dig blocks */
+	bool			gpu_vm_support; /* supports display from GTT */
 	int			disp_priority;
 	const struct amdgpu_display_funcs *funcs;
 	const enum drm_plane_type *plane_type;